The present invention relates to cap means for preventing resin from remaining in a mold of a runnerless injection molding apparatus. By decreasing resin remaining in the mold as less as possible, any resin sensitive to the plasticity temperature may be molded smoothly.
A conventional mold 1 of a runnerless injection molding apparatus is disclosed in FIG. 1. According to this example, a plurality of cavities 2 are disposed in the mold 1 in order to produce a large number of small-sized molded articles in one injection shot. A certain quantity of fused resin passes through a runner 3. The resin therein is fused by heater means 4 built into a heating cylinder 7. A pointed heating tip member 6 is fixed with the heating cylinder 7 to heat the area of a gate 5. Each heating cylinder 7 comprising the heater means 4 and the pointed tip member 6 is disposed in a preferred position of the mold 1 opposing to each cavity 2. A flange 7a of the heating cylinder 7 is fixed with the mold 1 by means of a runner plate 8 and screws 9. The runner plate 8 is provided with a bushing 10 for contacting a cylinder nozzle of an injection molding machine. A sprue 11 which passes through the bushing 10 is communicated with each gate 5 by way of the runner 3. Numeral 12 is a cartridge heater to constantly heat and fuse the resin in the runner 3 formed in the plate 8. The pointed tip means is particularly to heat and fuse the resin in the gate 5 which was cooled and solidified locally at the mold opening time.
In such a conventional runnerless type mold, the resin in the runner 3, which is a resin flow passage of the heating cylinder 7, is constantly heated to a desired temperature by the heater means 4 and place in a fused condition. Accordingly, when the mold is opened, there is no solidified sprue or runner taken away together with the molded articles. However, as shown in FIG. 2(a), since a space 13 is formed in the mold 1 enclosed by a periphery 7a of the heating cylinder 7 and the pointed tip member 6, the resin injected from the injection molding machine flows in an arrow direction A through the runner 3 in the heating cylinder 7 and is injected into the cavity 2 through the gate 5. Then, the disadvantage is that a certain quantity of resin will remain in the space 13 shown by dotted lines in FIG. 2. The remaining resin is inclined to increase temperature inevitably because its heating time is prolonged due to heating of the heater means 4. Such a prolonged heating is very disadvantageous for the resin of non-flammability and self-extinguishing properties. Particularly, when resins such as polyvinyl chloride resin remain for a long time in the space 13, toxic gas may result due to thermal decomposition. Further, when changing the resin color, the resin of previous color remains in the space 13, thereby a newly changed resin color is forced to mix with the previously fused resin of a different color. As a result, the color of the newly colored resin becomes bad. Accordingly, unless the remaining resin is removed from the space 13, a color change is not available. Of course, the same phenomenon arises at the time when replacing a previously used resin with a different resin. The task of the present invention is to remove the aforesaid disadvantages of the conventional art.